We have studied structure and viscoelastic properties of poly(styrene-b-2-vinyl-pyridine)s and their deutrated counterparts in the disordered state near the order-disorder transition (ODT) both in bulk and in a common good solvent. This specimen is suitable for such study since both components have almost the same physical properties but still segregate at certain conditions. The results we have achieved are listed as follows.(1)We determined ODT for the block copolymer samples in wide ranges of molecular weight (10k to million) and concentration (10% up to bulk) by rheometry and small angle neutron scattering (SANS) measurements. The data in semidilute region could be explained by the theory which account for fluctuation effect. However, the transition between semidilute and concentrated regions was not clear.(2)We found that the elastic parameters obtained by dynamic mechanical analysis are a few times larger than those of components but the elastic parameters measured by steady shear flow become almost the same as those of components. The difference cannot be explained by rheological behavior alone. There are two possibilities ; one is flow-induced ordering and another is suppression of fluctuation by steady shear flow.(3)To clarify above behavior, we performed flow-SANS measurements at the corresponding condition and found that there is no flow-induced microdomain structure so that we concluded the above viscoelastic behavior can be explained by fluctuation effect and its suppression under steady shear flow.